This invention relates to a method for growing a single crystal of compound semiconductors.
Single crystals of compound semiconductors have a wide scope of applications, e.g. substrates for integrated circuits, light emitting diodes or various kinds of detecting devices. According to the desired application of the semiconductor, semi-insulating, n-type or p-type single crystals are grown.
In this specification, compound semiconductors are the compound semiconductors of groups III-V and groups II-VI on the periodic table. The compound semiconductors of groups III-V are InP, InAs, GaAs, GsP, InSb, GaAb, etc. The compound semiconductors of groups II-VI are ZnSe, CdTe, ZnS, etc.
Conventional methods for growing a semiconductor crystal will be explained. Among several methods of manufacturing single crystals of compound semiconductors, liquid encapsulated Czockralski method (LEC) and horizontal Bridgman method (HB) are most preferable for industrial production.
The horizontal Bridgman method is a boat method which grows a single crystal in a boat by moving a temperature distribution along a horizontal direction.
The most powerful method among the category of pulling methods is LEC method, which grows a single crystal by pulling up a seed crystal from a crucible containing a compound melt covered by liquid encapsulant.
There is a problem in manufacturing compound semiconductors of groups III-V in that it is difficult to make a stoichiometric single crystal, because of the high dissociation pressure of the elements of group V.
In the LEC method in order to prevent the elements of group V from escaping, the material melt is covered with a liquid encapsulant which is pressed by inert gas or nitrogen gas at a high pressure. The liquid covering the material melt is called liquid encapsulant. The liquid encapsulant can effectively prevent the escape of the elements of group V. However a portion of the elements volatilizes and passes through the pressurized liquid encapsulant.
The liquid encapsulant must be strongly pressurized. Thus the whole apparatus is enclosed by a pressure vessel, and inert gas or nitrogen gas is filled in the pressure vessel up to several tens of atmospheres.
Because dense gas is filled in the pressure vessel, forcible gas convection occurs. The atmosphere in the vessel is thermally unstable because of the forcible convection. Therefore the single crystal is forcibly cooled immediately after it is pulled above the melt. Therefore thermal stress is apt to occur in the grown crystal. Strong thermal stress heightens the dislocation density of the crystal. This is a serious defect in the conventional LEC method.
A novel method belonging to a category of pulling methods for crystal growth has been proposed. It is called the vertical vapor-pressure-controlling method. Unlike the LEC method wherein only the area near the crucible is heated, in the vapor-pressure-controlling method, the whole of the pressure vessel is heated from top to bottom. Therefore the temperature gradient along the vertical direction is much smaller than that in the LEC method. It uses no liquid encapsulant. Material melt in a crucible contacts and keeps equilibrium with the gas filled in the pressure vessel. The pressure of the filled gas is low.
Furthermore the gas filled in the vessel is not an inert gas or a nitrogen gas, but a gas of an element of group V. A lump of an element of group V is placed at a pertinent position in the vessel. It is heated up to a pertinent temperature which realizes a desirable vapor pressure of the element of group V in the vessel.
Because the vapor pressure of the element of group V keeps equilibrium throughout the vessel, the vapor pressure of the element of group V in the material melt in the crucible is determined by the vapor pressure at the lowest temperature in the vessel.
Because this method is one of the pulling methods for crystal growth, this method grows a single crystal by dipping a seed crystal into a material melt and pulling a single crystal in succession to the seed crystal.
The pressure in the vessel is not so high as previously mentioned. In many cases, the pressure is determined to be 1 atm or slightly more than 1 atm. The vertical vapor-pressure-controlling method would be similar to an imaginary method which would be obtained by erecting the horizontal Bridgman method. The temperature of the upper space in the vessel is kept at about 650.degree. C. in the case of GaAs crystal growth.
Keeping the equilibrium of the vapor pressure in the vessel and preventing volatilization of the elements of group V from the melt and the crystal grown, the method pulls up a single crystal. Then the method can make a stoichiometric single crystal.
The methods described are applicable to the crystal growth of compound semiconductors of groups II-VI.
In principle the newly-proposed vertical vapor-pressure-controlling method is an excellent method. However the gas filled in the vessel is only the gas of the element of group V without inert gas or nitrogen gas. The gas pressure is much smaller than that in the LEC method. The fluctuation of vapor pressure is considerably large. The large probability of fluctuation makes it difficult to control the vapor pressure of the element of group V in the vessel.
The vapor pressure of the element of group V in the vessel is controlled by heating the coldest part of the vessel at a pertinent temperature. A slight change of temperature, however, causes a large change of vapor pressure of the element of group V. Subtle pressure control is difficult. Therefore the elements of group V are inclined to volatilize from a material melt or a grown crystal. Thus, the vertical vapor-pressure-controlling method has defects in that it requires complex apparatuses, and operations of the apparatuses are difficult. These difficulties would appear when compound semiconductors of groups II-VI are made by this method.